Pearlescent pigments which contain titanium dioxide in the top coat or which are composed of particulate TiO2 have a certain photocatalytic activity. If UV radiation then acts on a pearlescent pigment in the presence of water and oxygen, the UV activity of the pearlescent pigment can induce an accelerated breakdown of organic compounds, e.g. of a binder matrix. Even the proportion of UV contained in daylight can bring about this reaction. That is to say, for applications such as automobile lacquers, which are directly exposed to weathering, specially stabilized pearlescent pigments must be employed. In order to counteract this photocatalytic effect, which is deleterious to external applications, pearlescent pigments can be provided with various protective coatings for decreasing the photoactivity. These protective coatings often contain oxides of the elements cerium and/or zirconium, which have proven to be particularly effective.
In order to make thus stabilized pearlescent pigments compatible with the respective binder for application, in particular, in water-based pigmented coating systems, these pigments are provided with an organic aftercoat. This binds, on the one hand, to the surface of the pearlescent pigment and, on the other hand, to the binder. By this means, good condensation water resistance and good rheological properties, for example good dispensability, are obtained in the coating system. Moreover, by means of the organic aftercoat it is possible to positively influence the orientation behavior of the pearlescent pigments in the application medium and thus also their optical properties.
In EP 0 141 174, pearlescent pigments with improved weathering resistance are described, which have a protective coating which essentially consists of a rare earth metal compound—e.g. cerium—and a polysiloxane. Moreover, zinc or aluminum salts or alternatively silicate can be present in the protective coating, which is prepared in an aqueous suspension. The coating operation is carried out in aqueous suspension and the product is dried following isolation thereof.
EP 0 342 533 discloses pigments coated with zirconium oxide, to which a layer consisting of a hydrated metal oxide of the elements cobalt, manganese or cerium can be applied. Although the pigment treated in this way should now be readily employable in nonaqueous lacquer systems, it is, however, still unsuitable for water-dilutable pigmented coating systems according to EP 632 109, as in this case the formation of microfine bubbles is caused in the coated film.
According to the teaching of EP 0 632 109, a three-layer protective coat is applied to a platelet-shaped substrate coated with metal oxides. In a first stage, SiO2, in a second stage a hydroxide or hydrated oxide of cerium, aluminum or zirconium, and in a third stage at least one hydroxide or hydrated oxide of cerium, aluminum or zirconium and an organic coupling agent are applied. Moreover, the coupling agents must be hydrolyzed before binding to the pigment surface, and according to the teaching of WO 98/13426 only a maximum of 60% of the added coupling agents can be bound to the pigment surface.
WO 98/13426 discloses modified pearlescent pigments based on a platelet-shaped substrate coated with metal oxides. According to the teaching of WO 98/13426, the top coat consists of at least two oxides, oxide mixtures or mixed oxides of silica, alumina, cerium oxide, titanium oxide, or zirconium oxide and a water-based oligomeric silane system. The water-based oligomeric silane system is partially hydrophobized by alkyl radicals containing not more than eight carbon atoms. Nevertheless, the predominant proportion of the aftercoat is of a hydrophilic nature. Reading contrary to the disclosure, the pigments still have a certain swelling behavior in the condensation water climate test.
Furthermore, no investigations on the influence of the order of the oxide protective layers as regards their effectiveness in improving the UV stability of the pearlescent pigments are disclosed. Therefore an optimum protective layer architecture is also not described.
EP 0 649 886 relates to pearlescent pigments with a titanium dioxide or iron oxide coating, which are aftercoated in aqueous phase with a combination of hydrated cerium and aluminum oxides and subsequently dried.
According to the teaching of EP 1 203 795, a pearlescent pigment can have a layer system comprising hydrated oxides of silicone or aluminum in a first layer and hydrated oxides of silicone, aluminum, zirconium, or cerium in a subsequent second layer, wherein the composition of the first layer differs from that of the second layer. The pearlescent pigment further has a third layer comprising at least one organic hydrophobic coupling agent, wherein the organic hydrophobic coupling agent does not react with the binder of, say, a pigmented coating system. Contrary to the disclosure of this specification, the pigments do not show good adhesion in the condensation water climate test, as the hydrophobic coupling agents do not have suitable groups for binding to the coating system.
In most of the processes used in the prior art, SiO2 and/or alumina is applied as a first layer. A cerium oxide layer is usually applied subsequently thereto or together with further components as a mixed oxide deposition by precipitation. Binding of the silanes then generally takes place in a mixed precipitation with precipitation of the metal hydroxides in aqueous solution. On account of the mixed precipitation of the hydroxides and of the silane system, the effectiveness of surface coating with the oligomeric silane system is low. Consequently, excessively large amounts of the expensive silanes are employed, which unnecessarily increases the raw material costs.
In EP 1 084 198 B1, effect pigments are described which on account of their surface modification with reactive orientation agents show very good adhesion to the basecoat. However, viewed against this background, EP 1 084 198 B1 does not disclose any weather-stable pearlescent pigments.
In DE 103 19 937 A1, organofunctional surface-coated metal oxides are described. The metal oxides are surface-modified with α-silanes. However, pearlescent pigments and the requirements associated therewith are not mentioned in any way.